Lighting control devices must consume a certain amount of electrical power in order to perform their functions, such as occupancy sensing and supplying power to lighting loads. Often referred to as “phantom loads” or “vampire loads”, this lighting control device power consumption can be considerable in large buildings. For example, if two thousand occupancy sensors can save 20 milliamperes of electrical current at 24 volts (direct current), or 0.48 W each, then 960 W (0.96 kW) can be saved. Given inefficiencies in low voltage power supply design, the savings at the mains power supply will be higher (10-20% higher). Multiplied over thousands of buildings in a geographic area, power savings can be substantial.
Lighting control devices such as occupancy sensors often include an ambient light level sensor that measures the amount of ambient light. It is common for the ambient light level sensor to monitor the ambient light level to determine if an adequate amount of light is available in the space so that it does not need to turn lights on if occupancy is detected. Light may come from a window or skylight or from other electric lights not controlled by a particular lighting control device. Typically, the ambient light level sensor allows the user to adjust the ambient light level threshold at which this determination is made.